This invention relates to a cylinder used in an artificial leg for a person having upper thigh cut and an upper thigh stub. Typical examples of this type of artificial leg and cylinder used therein are disclosed in the Japanese patent publication No. S52-47638 and the description of prior art will be made below in connection with these examples.
As shown in FIG. 1, an artificial leg has upper and lower leg members 1 and 2 pivotably coupled through a joint 3 and a cylinder 4 is bridged therebetween at pivots 5 and 6 to support the joint of both members 1 and 2. The cylinder extends and contracts with straightening and bending of the joint caused by a walking action and serves to suitably brake and buffer the straightening and bending motion of the joint by means of internal air pressure.
As shown in FIGS. 2 and 3, the prior art cylinder comprises a cylindrical sleeve 10 having both ends closed with a head cover 12 and a rod cover 14, and a piston 16 fixed to a piston rod 18 penetrating through the rod cover 14 and fit slidably in the sleeve 10. The rod 18 has a coupling 20 to be coupled to the pivot 5 of the upper member 1 at an end thereof and the head cover 12 has a coupling 22 to be coupled to the pivot 6 of the lower member 2. The cylinder 4 may be inverted to couple the coupling 20 to the lower member 2 and the coupling 22 to the upper member 1, if the circumstances admit.
The piston 16 partitions the inner cavity of the sleeve 10 into a "head chamber" 24 adjacent to the head cover 12 and a "rod chamber" 26 adjacent to the rod cover 14. FIG. 2 shows a cylinder extended to give the maximum volume of the head chamber 24 and FIG. 3 shows the cylinder contracted to give the maximum volume of the rod chamber 26. The piston 16 has a check valve 28 for connecting the head chamber 24 with the rod chamber 26 and the check valve 28 is arranged to allow an air flow from the rod chamber 26 to the head chamber 24 but stop a backward flow. Therefore, the cylinder is subjected to large resistance to contracting less resistance to and the joint of extending the artificial leg straightens easily but bends awkwardly. As shown, the piston rod 18 includes a throttle valve 30 having a needle valve body 32 and connecting the head chamber 24 to the rod chamber 26, and the valve body 32 is moved forward and backward by an adjusting screw 34 to change the aperture of the throttle valve 30. Therefore, the contracting resistance of the cylinder or the bending resistance of the joint can be adjusted by adjusting the throttle valve 30.
As shown much better in FIG. 3, the piston rod 18 is partially enlarged in its diameter adjacent to the piston 16 and this portion will be referred to as "major diameter portion" 36. The penetration hole formed in the rod cover 14 for passing the rod 18 is partly enlarged to form a cavity 38 for receiving the major diameter portion 36 and an O-ring 40 having a V-shaped cross-section is disposed in the opening thereof. As shown in FIG. 2, the major diameter portion 36 enters the cavity 38 and forms a cushioning chamber 42 in the bottom of cavity 38 at the time of extension of the cylinder. Part of the air in the cushioning chamber 42 is discharged to the rod chamber 26 through a resistive path 44 which is schematically shown in a dashed line.
Referring next to FIG. 4 which schematically shows a human walking state, A1, A2, A3, A4, A5 and A6 denote head, back, waist, knee, heel and toe portions, respectively. While the drawing shows a cycle of walking after the heel portion A5 of one leg lands on the ground until it lands again thereon, which is decomposed into thirteen positions, in total, B1, B2, . . . B7 and C1, C2, . . . C6, the period from the position B1 and B7 in which the heel or toe portions A5 or A6 touches the ground is referred to as "standing phase" B and the period from the position C1 to C6 in which both heel and toe portions A5 and A6 are apart from the ground is referred to as "idling phase" C. At the time of walking, when one leg is in the standing phase B, the other leg is in the idling phase C, and the walking is effected by repeating the standing and idling phases, alternately.
At the position B1, the upper and lower leg 1 and 2 are substantially in line and the head and back portions A1 and A2 are a little behind the heel portion A5. From this position B1 to the position B4, the upper and lower leg 1 and 2 rotate about the heel portion A5 as they are in line and, at about the same time as the toe portion A6 comes in contact with the ground at the position B4, all portions from the head portion A1 to the heel portion A5 are substantially straightened into an erected state. While these portions from the head portion A1 to the heel portion A5 fall gradually forward as they are nearly in line from the erect positions B4 and the position B7, the upper part of the body begins to rise about the waist portion A3 after about the position B7, and the upper and lower leg 1 and 2 rotate before as they are before about the toe portion A6. Next, after the position C1, the toe portion A6 leaves the ground and the lower leg 2 begins to rotate in clockwise direction about the waist portion A3 with respect to the upper leg 1 or, in other words, bending of the knee A4 begins and, at the position C3, the bending becomes maximum. In contrast, from the position C3 to C6, the lower leg 2 rotates in counterclockwise direction about the waist portion A3 with respect to the upper leg 1 and, at the position C6, they reach into a nearly straightened state.
The cylinder 4 operates as follows during each cycle of walking as described above. During the standing phase B in which the upper and lower leg members 1 and 2 are nearly in line, the cylinder 4 is substantially in the extended state as shown in FIG. 2. After the position C1, however, it begins to contract and the air in the head chamber 24 flows through the throttle valve 30 into the rod chamber 26. By adjusting this air flow by the needle valve body 32, the swing-up speed of the lower leg member 2 can be adjusted adequately. At the maximum swing-up position C3 of the lower leg member 2, the cylinder 4 arrives at the contracted state as shown in FIG. 3 and the air remaining in the head chamber 24 is compressed in a depression 46 formed in the head cover 12. With a repulsive force of this compressed air, the cylinder 4 begins to extends thereafter to commence swing-down of the lower leg member 2. Then, the air in the rod chamber 26 flows through the check valve 28 into the head chamber 24 and, therefore, the lower leg member 2 is smoothly swung down. Finally, at the position C6, the lower leg members 1 and 2 are straightened completely and the cylinder 4 is in the extended state of FIG. 2, in which the major diameter portion 36 fits in the cavity 38 to form the cushioning chamber 42. The cushioning chamber 42 serves to reduce shock when the upper end 2a of the lower leg member 2 butts against the lower end 1a of the upper leg member 1.
In the above-mentioned prior art cylinder, however, the shock reducing effect of the cushioning chamber 42 is relatively small since its volume is small and the air therein is rapidly compressed. Especially, when swing-down speed of the lower leg member 2 is high, the above-mentioned shock cannot be absorbed completely and, therefore, the wearer of the prosthesis is subjected to an unpleasant feeling due to this shock when he increases his walking speed. Even if the walking speed is relatively low, there should be the same problem when the artificial leg is long and heavy.
Accordingly, an object of this invention is to provide an improved cylinder for artificial legs, which has a large cushioning chamber for completely absorbing the above-mentioned shock to enable comfortable walking regardless of the walking speed and the weight of the lower leg member.